In comparative studies of the behavioral and biological effects of two structurally-related monoamine oxidase (MAO) inhibitors, clorgyline was found to be a highly selective inhibitor of MAO type A during longer-term administration in man. Pargyline had greater inhibitory effects on MAO type B, but also partially inhibited MAO-A in the doses used. Preliminary behavioral data indicate greater antidepressant and antianxiety effects with clorgyline than pargyline, although both drugs had activating effects. In other studies, the effects of these drugs and of another MAO inhibiting antidepressant, tranylcypromine, on the diurnal secretion of plasma cortisol, growth hormone and prolactin and on the sleep EEG were examined in depressed patients.